Gas heater



H. N. STONE Feb. 23, 1954 GAS HEATER Filed May 1, 1952 RE PLENISH MENTOF CYC LED' GAS O F PARTICLES INVENTOR. H. NATHAN STONE A TTORNE) units.

Patented Feb. 23, 1954 GAS HEATER H. Nathan Stone, Worcester, Mass.,assignor to Norton Company, Worc tion of Massachusetts Application May1, 1952, Serial N 0. 285,578

20 Claims. 1 The invention relates to gas heaters. One object of theinvention is to provide gas heating apparatus which can be embodied in asmall compact unit. Another object of the in vention is to provideapparatus for rapidly heating gas by means of electric resistanceheating Another object of the invention is to extract larger quantitiesof heat units per unit volume of heater equipment, thus attaining highoutput of thermal energy from such, resistors. Another object is toachieve a high heat transfer coefficient from a resistance unit to gas.Another object is to provide an electric heater to heat gases thatdeleteriously affect the resistances used, without subjecting theresistances to such gases, while at the same tim heating a large volumeof gas to a high temperature by means of a reiatively small heater, andas an example I can use silicon carbide heater bars which can beoperated at high temperatures for a long time if located in a protectiveatmosphere and heat a gas such as steam without deteriorating the heaterbars, whereas such silicon carbide bars have only short lives in a steamatmosphere. Another object of the invention is to provide a heater withelectrical resistance heater units, preferably silicon carbide bars, andto provide an inert or non-oxidizing atmosphere in the chamber where thebars are located, to recycle the inert gas and to transfer the heatunits to another chamber or passage through which the gas to be heatedpasses.

Other objects will be in part obvious or in part pointed outhereinafter.

The accompanying drawing illustrates an embodiment of the invention inwhich most of the heater is shown in vertical section, with certainassociated apparatus and piping being illustrated diagrammatically,

Referring now to the drawing, it will facilitate a quick understandingof the invention first to trace the path of the inert or non-oxidizinggas and then to trace the path of the gas to be heated. The inert ornon-oxidizing gas enters a recuperator i at entrance port 2. Therecuperator I may be of any desired construction and as recuperators arewell known I have simply given the diagram found in the current Rules ofPractice of the United States Patent Oiiice. It will, however,preferably be a large recuperator or a series of recuperators greatly tolower the temperature of the gas. The gas which is now cooled leaves therecuperator I through port 3 and goes to a blower 4 which is alsodiagrammatically illustrated. The gas is taken from the ester, Mass., acorporablower i to a pipe 5 which is connected to a pipe 6 by a T-union7. The pipe 6, shown as a. straight pipe, is coupled to a bent pipe 8which is coupled to a straight pipe 8a which extends into a fluidizingchamber 9 having therein resistor rods or bars 10, for example the wellknown silicon carbide resistor bars having cold ends H. The gas which isheated in the fluidizing chamber 9 to a high temperature is taken by arefractory pipe 12 back to the entrance port 2 of the re-- cuperator I.In the drawing, which is diagrammatic, the entrance port 2 is shown aslocated at some distance from the refractory pipe 12 but in reality theport 2 may be only a short distance from the top of the chamber 9 andthus the pipe 12 can be a short straight pipe.

The gas to be heated, which is under pressure so that it will movethrough the heater, enters the recuperator l at an entrance port i 3 andleaves the recuperator l at an exhaust port I4, having been heated to anintermediate temperature. The partially heated gas then enters theheater at a pipe coupling 26 and the gas then enters a refractory pipe2i leading to a refractory T-union 22 which is connected to a refractorypipe 23 connected to a curved refractory pipe 24 connected to a straightrefractory pipe 24a which leads to a fluidizing chamber 25, and thefully heated gas then exhausts through a reiractory pipe 26 to anyapparatus for any process of conversion with which this invention is notconcerned. In accordance with my invention gas can be heated to hightemperatures economically in a small apparatus, and in industry andchemical engineering there are many reasons for heating gas to hightemperatures and so there will be many uses for my apparatus but it isunnecessary for me to describe these or any of them as they relate toother arts.

The chambers 9 and 25 contain a quantity of fine particles 28 ofrefractory material in fluidized condition. The refractory material maybe an oxide, carbide, silicide, nitride, boride, or a mixture ofcompounds. Just what refractory material to select depends upon whichare poisonous to Whatever reaction is to take place and it also dependsupon availability and cost. Furthermore, at the temperatures involved,the nature of the gas entering the pipe 2| and present in the chamber 25has to be considered. If this is reducing it might be desirable to avoidsome of the oxides if the temperature is too high; if the gas isoxidizing it might be desirable to avoid some of the carbides. Readilyavailable refractory oxides are alumina, silica and gamer,

magnesia; the most readily available and inexpensive carbide which issufficiently refractory for most applications is silicon carbide whichwill probably be preferred in most instances. The borides, silicides andnitrides are less available and more expensive but some thereof may bepreferred for particular applications.

The particle size of the refractory material is a matter for carefulconsideration. In general the finer the particle size of the fluidizedmaterial, the more efficient is the transfer of heat from the chamber 9to the chamber 25 and therefore, for a given temperature of the gasexhausting from the refractory pipe-2's,- the higher 'can b'e the rateof flow of the gas being he'ate'd. -However for several reasons it isundesirable to lose particles in great quantities; it isex-pensive itrequires cleaning of any apparatus coupled to the heater, and it mayinterfere with somereactions. Accordingly since particles of the finersizes will be carried away through the pipe 25 and lost to the heater, Ifind itis in general desirable to use particles not finer thanlllOgritsize. On the other hand for efilcient use of the heater the particles ingeneral should be .no coarser than 60 grit size. So therefore the bestspecification is that the particles be through No.

*80 screen onto No. 100 screen. :However'this'is 'no hard and fast rulebecause for some applications loss of particles would be unimportant butrate ofheat transfer'might be very important "hence there is no reallimit- .to the fineness of the particles which can be employed althoughfor practical purposeslI can say that particles finer than 600 gritsize, U. S. Bureau of Standards, probably would not-be used. At theother end of the scale particles coarser than 24 grit size would notappear to 'beuseful inthisinventi-on.

The fluidized condition is really a state of gaseous-solid emulsion.chamber 9 from the pipe 8a sustains the particles .28 of refractorymaterial inthe chamber which cannot choke the entrance port 29 becausethe velocity of the gas stream prevents them from doing so. bottom ofthe chamber ii'where the velocity of the gas stream is inadequate tosupport them and forma funnel shaped bottom for the chamber 9.

"Fine particles stay in "this gaseous-solid emul- "sion condition knownas the fiui'd'iZed condition wherever the velocity of the gas stream ishigh enough to keep themsu'stained or in motion. A fluidized emulsion ina chamber such as the chamber 9 where the gas stream is mainly upwardlywill reach a levelanalogous to'a liquid level. I provide refractoryoverflow pipes 32 and 32a and provide a-greatenough quantity ofrefractory particles to create a fluidized level 33-above the level ofthe overflow port 34 which is the entranceto the pipe 32. The pipes 32and 32a extend downwardly and the latter is connected to the refractoryT-union'22. Particles will therefore fiow downwardly through the pipes32 and 32a to the T-union 22 where they will be picked up by the gaswhich is moving in a'fast stream to the right in the pipes 21 and 23'and'through the T-union 22. The particles 28 will therefore 'be carriedintothe fluidizing'chamber 25 which has stagnant particles 35 forming afunnel shaped bottom at the lowest point of which is the entrance port35 for the gas and the fluidized par- -ticle's. Again I provide enoughparticles 23and -so adjust the sizes of the chambers-9 and 25 and therates of fiowof thetwo gas streams that the The gas entering thestagnant particles 39 settle inithe the tube. what changed. Those solidsin contact with the 4. fluidized level 31 in the chamber 25 will beabove an overflow port 38 of a refractory pipe 39 extending downwardly.Particles 28 will therefore descend through the pipe 39 and a pipe 39aconnected to it to be picked up by the gas incoming through the pipe 5and going through the T-union 1, through pipe 6 and through the pipes '8and 9'a-'into the chamber 9. 'Thus the cycle is completed and "theparticles "28 travel continuously through chamber 9, pipes 32 and 32a,union 22, pipe 23, pipes at and E ia, chamber 25, pipes 39=and 39a,union 1, pipe 6, pipes 8 and 8a back again to chamber 9. During theoperation of the heatenbothf chambers 9 and 25 should have fluidized.particies 28 of refractory material therein up "tojust above theoverflow ports.

The fluidized particles 23 in the chamber 9 are heated by the resistorrods NJ. The illustrative embodiment is of four rods Iii, two havingaxes in the plane of the section, the middle one being near the far sideof the chamber 9, and there being a *fourthone, not shown, in front ofthe plane of the section; thus'the rods l6 are "symmetrically located inthe chamber 9. One or -more resistor rods H3 deliver more heat units pertminute to the fluidized particles than they would to the walls of thecharnber9 and thus the in- .troductionof the bed ofifiuid'ized solidsmakes it possible to extract much more heat fromthe r-esistor rods in agas heater'of'a given sizethan .could be extracted in the absence of thefluidized particles. This can best be illustrated by these examples:

Example-I 'A vertical tube has "several vertical resistor rodsdistributed-"within it and a non-radiating gas is passed upward throughthe tube in order topick up heat from the resistor rods. The

'heat transfer mechanisms are convection and conduction and the areas oftransfer simply the surface areas'o'fthe rods and the tube which absorbsradiant energy from the rods and passes it onto the gas by convectionand conduction.

The heat transfer coeficient is controlled by the "gas velocity andbecause of practical limitations the velocity is confined to a range ofvalues which provides extremely low heat transfer coefficients. This is'primarily due to theexistence of slow moving .gas films along theheating surface wherein the heat transfer is purely by conduction andgases have low heat conductivities.

Example If This case is the same'asEXa'mple I except that a'static bedof granular solids-is introduced into The mechanism of transfer is some-Example III I This case is'the same as'EXample II "except that thegasveloc'it-y is increased to' the point "where fiuidizat-ion isobtained. The mechanism of heat transfer is th'e sameasin'Example II butnow the mechanical turbulence brings about a uniform temperaturethroughout the bed. The heat transfer area is greatly increased andthough the overall gas velocity may be low, the localized gas velocitybetween the particles is high, and so the slow moving gas films arereduced to a minimum. I'his results in an overall heat transfercoefficient many times larger than that available in either Example I orII.

Regardless of how carefully the grit was screened to eliminate finessome fines will necessarily be found among the particles 28 when theapparatus is first charged therewith. Most of these will soon pass outthrough the pipe 26. Thus after the apparatus has been in use for ashort time there is very little loss of fluidized particles butoccasionally a particle will gain the velocity of escape and be lost.Therefore if calculation is made for the initial loss the heater willoperate for a long time without replenishment of particles buteventually replenishment has to be made. Any closable opening in thepipe 5 will serve as an entrance for replenishment of particles.Replenishment has been indicated diagrammatically in the drawing andpreferably is done through a pipe which opens at a level above the level31 as indicated, as in such case the blower 4 does not have to bestopped during replenishment.

The constructional details can be widely varied but as conducive to afuller understanding of the invention the further features of theapparatus herein illustrated will be briefly described. The chamber 9comprises a cylindrical steel casing 4! having a steel bottom 42 andhaving a refractory lining 43 which can be simply packed refractorygrain and for heat insulation zirconia is preferred. Inside of thelining 43 is a cylindrical lining 44 made out of shaped refractorybricks, such as sintered alumina bricks. refractory cap 45 is made outof any suitable refractory material such as a single piece of sinteredalumina. Braided wire conductor ribbons 41 are wrapped around the coldends I l and held in place by spring metal clips 48 and are The bottom42 is part of a horizontal frame piece 42 supported by legs 50, 5! and52. This piece also forms a bottom to a cylindrical steel casing 53forming the supporting structure for the chamber 25. A refractory lining54 similar to the lining 43 and a cylindrical refractory lining 55similar to the lining 44 complete the chamber which has a removablerefractory cap 56. The refractory pipe I2 passes through the cap '45 andthe refractory pipe 26 passes through the cap 56 as shown and a littlecement on top of the caps and 56 can be used to hold the pipes in place.A refractory bottom plate 51 supported by the steel bottom 42 supportsthe cylindrical refractory lining and the stagnant particles 55.

' A steel pipe 6| surrounds the refractory pipe 6, a steel pipe 62surrounds the refractory pipes Band 8a; a steel T-union 65 encompassesthe pipe 2| and the T-union 22; a steel pipe 66 and a curved steel pipe61 surround the pipes 23, 24 and 24a and in every case refractory grainis rammed between the refractory pipe or union and the steel pipe orunion. Similarly a steel T-union 69 surrounds the T-union I and this isconnected A removable to a steel pipe 70 surrounding the pipe 39a belowthe bottom 42. Steel casings H and 12 welded to the sides of the casings4| and 53 respectively encompass the upper parts of the pipes 32a and39a respectively and also parts of the pipes 32 and 39. Refractory grainis rammed inside the union 69, the pipe 16 and the casings H and 12. Thepipes 6| and 66 are secured to the leg 50 by brackets 15 and 76. Theresistors 50 should not fit tightly in each of the cap 45 and bottomplate 42 as theywould be fractured due to clon gation and contraction ifthey did fit tightly in each of these parts; they are shown as locatedin oversized holes in the cap 45 and bottom plate 49 and hence I providea refractory plate 18 to support the rods H! which also supports arefractory sleeve 2'9 surrounding and providing thermal insulation forthe pipe 8a which is held in place by the plate 18 through which itpasses. The plate 18 is in turn supported by a steel plate which issupported by bolts 8| extending downwardly from the steel bottom 42.

The gas to be cycled through the recuperator l, blower 4, pipe 5 etc.fluidizing chamber 9 and back to the recuperator can be any of the inertgases of which helium and argon are the most readily available. Argonhas distinct advantages in that its specific gravity is greater andhelium diffuses rather readily but is fairly inexpensive at the presenttime. Nitrogen can be used in some applications as it is inert towardssilicon carbide or metals at the lower range of temperatures.

The resistor rod is is made of recrystallized silicon carbide accordingto a general process invented by Francis A. J. Fitzgerald, see forexample U. S. Patent No. 650,234, dated May 22, 1900. This process wasdeveloped in Switzerland for the manufacture of resistors about thirtyyears ago and such resistors are now well known. The cold ends I l aremade by impregnation with silicon as described in U. S. patent to HenryNoel Potter No. 1,030,327 of June 25, 1912. The silicon impregnatedsilicon carbide has far lower resistivity than the remainder which issimply recrystallimd silicon carbide and so therefore the voltage dropoccurs between the cold ends and the heat is liberated between the coldends. The central portions of these resistors, i. e. the portionsbetween the cold ends, are necessarily porous and are readily attacked,at the usual temperatures of operation, by such gases as oxygen, steamand to a lesser extent by air. At such usual temperatures oxygen quicklyoxidizes'silicon carbide and steam appears to have a strong oxidizingeffect thereon also. While these resistors can be and have been operatedfor long lengths of time in an air atmosphere it has usually beenconsidered that they shouldnt be run at temperatures much over 1400 C.if they are going to have reasonable life expectancy. By operating themin an inert atmosphere such as A or He, they can be heated totemperatures greater than G C. even up to 1600 C. and will usually lastlonger than the same resistors in air at 1400 C. Nitrogen will nitridethe silicon carbide resistors at very high temperatures, but they can beoperated at 1400 C. in nitrogen for a much longer time than they can beoperated in air and will have useful lives in nitrogen at 1500 C. oreven higher. To keep the cold ends I I from being burned out I mayfurther form them on enlarged end portions 82 providing temperaturegradients between the ends I l and the central hot portions of theresistors.

acre-n25 There, aremany reasons for wanting to heat air, steam andoxygen (and nitrogen at the highertemperatures) and in this heater theycan be heated. without affecting the life of the resistors, Even at 1000C. silicon carbide resistors would. quickly burn out in oxygen. In steamat evenl200 C. the silicon carbide, resistors would have veryshortlives. Oxygen, steam and air are deleterious to metallic resistors athigh temperatures so the same reason exists for this apparatus usingmetallic resistors.v In fact, molybdenum, which has a. melting point of2620 0., cannot be heated; to anywherev near that temperature in air; inthis apparatus, protected by argon or helium, it could be heated nearlyto the. meltin point, and it could be heated to reasonably hightemperatures in nitrogen- But the enumeration of certain. gases whichthe heater can heat: to particular advantage is not. meant to excludeothers. Any gas can. be introduced through the port 53' into recuperatorI into the pipes 25 and23 and; through the fluidizing chamber 25. Forexample the gas may be a hydrocarbon to be cracked and it would beundesirable to pass the hydrocarbon through a chamber containingsiliconcarbide resistance elements because of the deposit of carbonthereon. Furthermore there may be other gases which could be cycledthrough the blower 4 etc. For replenishment of the cycle gas as itislost through diffusion or in any other manner, it will sufiice to haveon hand a bottle. full of the gas under pressure with suitable valvesand a pipe connected to the line between the blower i and the pipe 5, asclearly indicated in the drawing.

In describing the invention I have necessarily described a completeheating system but the heater proper is the. unit which is shown indetail (minus the pipes l2 and 28) as it is such heater which is anarticle of commerce to be sold without the recuperator or the blower orthe outside piping (indicated by lines and arrows) which are otherarticles of commerce. The particles of refractory material to befluidized to wit: refractory grain is likewise a separate article ofcommerce. I desire therefore particularly to claim the heater propershown in detail as this is a manufacturing unit and a manufacture forsale.

"The reason for using the recuperator I with r the apparatus of thisinvention is that, if the heater is operated at high temperatures, ascontemplated, the ordinary blower (made ofmetal) would be, quicklyoxidized or even melted. By using thesystem illustrated and describedthe blower-t receives only moderately hot gas but the heat units in thegas exhausting from the chamber 9 are not entirely lost. In someapplica- 'tions",'where the temperatures are somewhatlower 'or'if theblower including its impeller and shaft are made of refractorymaterials, the recuperatcr I can be dispensed with entirely or be ofdimin .ished size and capacity for exchangin heat units.

The. selection of materials is important but dependsupon the-gas beingcycled, the gas being. heated, and the temperature of the. resistors it.Sintered alumina is a good material from which to make the variousrefractory-pipes, the cylindrical linings, the refractory caps and therefractory bottom plates. This material isresistant. to abrasion, it isnot a conductor of electricity and it will not react excessively withthe fluidized particles 28 evenif they are carbide particles at thelower range of temperatures, say up to about 1450" C. For operatingtheapparatus at resistor temperatures of 1450" C. and lower, I thereforerecommend that the refractory pipes, refractory T-unions, refractorycaps and refractory bottom plates be made of sintered alumina and thiscan be so whether the cycled gas is argon, helium or nitrogen andassuming that the fluidized particles are silicon carbide particles andthe resistors it are silicon carbideresisters. Sintered alumina hasanother good characteristic in that it is relatively impervious to gasesalthough if helium is used there will be some gas lost throughdiffusion. To prevent gas loss through the cap as and the bottom plate49 asbestos packing 8t is'provided around the cold ends ll of theresistors ill. Sintered alumina cannot be classed as a thermal insulatorbut it is a poor conductor of heat but the provision of the refractorylinings s3 and 54 of zirconia particles; provides good heat insulationfor the apparatus. The refractory grain rammed between the: refractorypipes and unions and the steel pipes and unions should likewisepreferably be zirconia for the best results. I do not recommend zirconiabricks for the cylindrical linings 34- and 55 because at hightemperatures zirconia becomes conductive.

Another selection of materials useful for resister temperatures up to1600 C. comprises zirconia grain linings throughout, as in the previoneembodiment, silicon carbide fluidized particles 2E and refractory pipes,T-unionaocaps and bottoms of bonded silicon carbide with a highlyrefractory bond. While recrystallized silicon carbide is electricallyconductive, bonded silicon carbide is not. In this. embodiment thematerial everywhere is silicon carbide, only the solid pieces have aminor proportion of refractory bond of a nature lmown to those skilledin the ceramic arts. Thus there will be no reaction etween the.fluidized particles and the walls of any of the pipes or chambers orbetween the particles and the resistor rods. For many practicalapplications this is probably the best selection of materials;

In the claims the fiuidizing' chamber 9 is referred to as a firstchamber, the fluidizing chamber 25 is referred to as a second chamber,the T-union i is referred to as a first T-union, the T-union 22 isreferred to as a second T-union, the pipes 32 and 320: are referred toas a first pipe, the pipes 39 and 35a are. referred to as a second pipe,the pipes 5, 8 and 8a are collectively referred to asa third pipe andthe pipes 23,. 24 and 24a are referred to as a fourth pipe. This s isbelieved to be. necessary for identification'without confusingcircumlocution since there are pluralities of chambers, T-unions andpipes. The expression T-union is to be taken to include any gasconnection having three branches and pipe" means any conduit capable offunc tioning to convey gas as indicated herein.

it will thus be seen that there has been profvided by this invention aheater in which the various objects hereinabove set forth together withmany thoroughly practical advantages are successfully achieved. As manypossible embodiments might be made of the above invention-and as manychanges might" be made in the embodiment above set forth, it is to beunder:- stood that all matter'hereinbefore set forth or shown in theaccompanying drawings is to be interpreted as illustrative and not. in alimiting sense.

I claim:

1.,Agas heating apparatus comprisinga first refractory lined fluidizingchamber, electrical resistance heating means in said chamber, a firstrefractory pipe having an opening into said chamber well above thebottom thereof and ex tending downwardly from said opening, chamberhaving an upper opening well above the level of the opening into saidchamber of the first refractory pipe, said chamber also having anopening in the bottom thereof, a second refractory lined fiuidizingchamber, a second refractory pipe having an opening into said secondchamber well above the bottom thereof and extending downwardly from saidopening, said second chamber having an upper opening well above thelevel of the opening into said second chamber of the second refractorypipe, said second chamber also having an opening in the bottom thereof,a third refractory pipe connected to the opening in the bottom of thefirst chamber, a fourth refractory pipe connected to the opening in thebottom of the second chamber, a first refractory T-union two of thebranches of which are connected respectively to the second refractorypipe and to the third refractory pipe, and a second refractory T-uniontwo of the branches of which are connected respectively to the firstrefractory pipe and to the fourth refractory pipe, whereby when theupper opening of the first chamber is connected to a blower, the bloweris connected to the first T-union, the second T- union is connected to asupply of gas to be heated and the chambers are partially filled withparticles of refractory material, the apparatus will function as a gasheater and the gas to be heated will not come in contact with theelectrical resistance heating means.

2. A gas heating apparatus comprising a first refractory linedfiuidizing chamber, electrical resistance heating means in said chamber,a first refractory pipe having an opening into said chamber well abovethe bottom thereof and extending downwardly from said opening, saidchamber having an upper opening well above the level of the opening intosaid chamber of the first refractory pipe, said chamber also having anopening in the bottom thereof, a second refractory lined fiuidizingchamber, a second refractory pipe having an opening into said secondchamber well above the bottom thereof and extending downwardly from saidopening, said second chamber having an upper opening well above thelevel of the opening into said second chamber of the second refractorypipe, said second chamber also having an opening in the bottom thereof,a third refractory pipe connected to the opening in the bottom of thefirst chamher, a fourth refractory pipe connected to the opening in thebottom of the second chamber, a first refractory T-union two of thebranches of which are connected respectively to the second refractorypipe and to the third refractory pipe, a second refractory T-union twoof the branches of which are connected respectively to the firstrefractory pipe and to the fourth refractory pipe, and a quantity ofrefractory particles in each of said fiuidizing chambers, said particlesbeing between 24 grit size and 600 grit size, whereby when the upperopening of the first chamber is connected to a blower, the blower isconnected to the first T-union and the second 10 T-union is connected toa supply of gas to be heated, the apparatus will function as a gasheater and the gas to be heated will not come in contact with theelectrical resistance heating means.

3. A gas heating apparatus as claimed in claim 2 in which the refractoryparticles are silicon carbide particles.

4. A gas heating apparatus as claimed in claim 3 in which the electricalresistance heating means comprises silicon carbide resistors.

5. A gas heating apparatus as claimed in claim 4 in which the refractorylining of one of the chambers is a silicon carbide lining.

6. A gas heating apparatus as claimed in claim 2 in which the electri alresistance heating means comprises silicon carbide resistors.

7. A gas heating apparatus as claimed in claim 6 in which the refractorylining of one of the chambers is a silicon carbide lining.

8. A gas heating apparatus as claimed in claim 2 in which the refractorylining of one of the chambers is a silicon carbide lining.

9. A gas heating apparatus as claimed in claim 8 in which the refractoryparticles are silicon carbide particles.

10. A gas heating apparatus as claimed in claim 2 in which therefractory lining of one of the fluidizing chambers is an aluminalining.

11. A gas heating apparatus as claimed in claim 10 in which therefractory particles are alumina particles.

12. A gas heating apparatus as claimed in claim 11 in which theelectrical resistance heating means comprises silicon carbide resistors.

13. A gas heating apparatus as claimed in claim 2 in which therefractory particles are alumina particles.

14. A gas heating apparatus as claimed in claim 13 in which theelectrical resistance heating means comprises silicon carbide resistors.

15. A gas heating apparatus as claimed in claim 2 in which theelectrical resistance heating means comprises silicon carbide resistorsand the refractory lining of one of the fluidizing chambers is analumina lining.

16. A gas heating apparatus as claimed in claim 1 in which theelectrical resistance heating means comprises silicon carbide resistors.

17. A gas heating apparatus as claimed in claim 16 in which therefractory lining of one of the chambers is a. silicon carbide lining.

18. A gas heating apparatus as claimed in claim 1 in which therefractory lining of one of the chambers is a silicon carbide lining.

19. A gas heating apparatus as claimed in claim 1 in which therefractory lining of one of the chambers is an alumina lining.

20. A gas heating apparatus as claimed in claim 19 in which theelectrical resistance heating means comprises silicon carbide resistors.

H. NATHAN STONE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,837,179 Benner et al. Dec. 15, 1931 2,246,322 Roth June 17,1941 2,536,099 Schleicher Jan. 2, 1951

